An endoluminal prosthesis is a medical device commonly known to be used in the treatment of diseased blood vessels. An endoluminal prosthesis is typically used to repair, replace, or otherwise correct a damaged blood vessel. An artery or vein may be diseased in a variety of different ways. The prosthesis may therefore be used to prevent or treat a wide variety of defects such as stenosis of the vessel, thrombosis, occlusion, or an aneurysm.
One type of endoluminal prosthesis used in the repair of diseases in various body vessels is a stent. A stent is a generally longitudinal tubular device which is useful to open and support various lumens in the body. For example, stents may be used in the vascular system, urogenital tract and bile duct, as well as in a variety of other applications in the body. Endovascular stents have become widely used for the treatment of stenosis, strictures, and aneurysms in various blood vessels. These devices are implanted within the vessel to open and/or reinforce collapsing or partially occluded sections of the vessel.
Stents are generally open ended and are radially expandable between a generally unexpanded insertion diameter and an expanded implantation diameter which is greater than the unexpanded insertion diameter. Stents are often flexible in configuration, which allows them to be inserted through and conform to tortuous pathways in the blood vessel. The stent is generally inserted in a radially compressed state and expanded either through a self-expanding mechanism, or through the use of balloon catheters.
A graft is another type of endoluminal prosthesis which is used to repair and replace various body vessels. Whereas a stent provides structural support to hold a damaged vessel open, a graft provides an artificial lumen through which blood may flow. Grafts are tubular devices which may be formed of a variety of material, including textile and non-textile materials. One type of non-textile material particularly suitable for use as an implantable prosthesis is polytetrafluoroethylene (PTFE). PTFE exhibits superior biocompatibility and low thrombogenicity, which makes it particularly useful as vascular graft material in the repair or replacement of blood vessels. In vascular applications, the grafts are manufactured from expanded PTFE (ePTFE) tubes. These tubes have a microporous structure which allows natural tissue ingrowth and cell endothelialization once implanted in the vascular system. This contributes to long term healing and patency of the graft.
It is also known to combine a stent and a graft to form a composite medical device. Such devices are often referred to as stent/grafts. Such a composite medical device provides additional support for blood flow through weakened sections of a blood vessel. In endovascular applications the use of a stent/graft combination is becoming increasingly important because the combination not only effectively allows the passage of blood therethrough, but also ensures the implant will remain open and stable. However, the graft can reduce the overall longitudinal flexibility of the composite device. Longitudinal flexibility is of particular importance to such stent/graft endoluminal prosthesis as the device must be intraluminally delivered through tortuous pathways of a blood vessel to the implantation site where the stent is expanded.
Several types of stent/graft inventions are known in the art. For example, U.S. Pat. No. 5,151,105 issued to Kwan-Gett discloses a collapsible textile vessel sleeve that is collapsible to a very small diameter in order that it may be placed in position within the abdominal or thoracic aorta by a catheter via the lumen of the femoral artery. Such a procedure obviates the need for a major surgical intervention, and reduces the risks associated with such a procedure. Other stent/graft composite devices using a fabric are shown in U.S. Pat. No. 5,628,788 to Pinchuck.
U.S. Pat. No. 5,575,818 issued to Pinchuk discloses polyurethane coatings which may be applied to a stent to form a stent/graft. The coatings disclosed may be bonded to the stent through the use of many different methods. For example, an inner lining or coating can be first spun on a mandrel, after which a stent covered with an adhesive substance is pulled down on the lining and then the adhesive is cured, melted and solidified or dried. Another alternative is to place the stent on a mandrel, apply (e.g., spray, dip, pad, etc.) a thin coating of polyurethane lacquer over the stent, and then spin the coating over the lacquer so that it is bonded to the stent once the lacquer dries.
One difficulty encountered in stent/graft structures which employ PTFE or ePTFE as the graft portion, is obtaining a proper bond between the stent, which is usually metallic or other material dissimilar to the graft portion. For example, U.S. Pat. Nos. 5,700,285, 5,735,892 and 5,810,870 to Myers et al. disclose the use of two ePTFE tubular sheets which are bonded together through the space between a stent sandwiched therebetween. These patents also disclose the use of fluorinated ethylene propylene (FEP) as an adhesive for bonding the stent to a tubular sheet or sheets.
It is well recognized, however, that few materials bond well to PTFE or ePTFE due to its chemical makeup. The surface of PTFE materials is difficult to wet and the relatively small pore sizes of ePTFE are difficult to penetrate effectively to obtain good mechanical bonds.
Thus, while ePTFE has shown to possess many desirous characteristics for use in conjunction with a stent, attachment of the polymeric tubular grafts to the stent has always presented its challenges. Due to the physical and chemical inertness of an ePTFE vascular graft, thus, it is difficult to adheringly attach such grafts to other structures. The present invention addresses inherent difficulty in bonding the stent.